MONDAY, April 12 (HealthDay News) -- Two new studies provide more evidence of a genetic link between an eye disease that affects millions of older people and the regulation of so-called "good" cholesterol in the body.

The researchers don't fully understand the connection, nor are they near finding better treatments for the eye disease, known as age-related macular degeneration (AMD). Still, they say the results could lead to future advances.

Scientists had been exploring the cholesterol-eye disease link for some time, noted Anand Swaroop, an investigator with the National Eye Institute and lead author of one of the reports. "But somehow, their studies did not get much attention. Hopefully now the genetic evidence is so strong that people will explore those possibilities in much more detail," Swaroop said.

An estimated 10 million or more people in the United States develop AMD each year, including about 25% of people over the age of 65 and 30% to 35% of those over 75, Swaroop said. The disease affects an area of the retina known as the macula, causing people to lose the ability to see in the central area of their field of vision.

"You cannot see straight," Swaroop explained. In the most severe cases, "you cannot drive, and you cannot do day-to-day things in life."

The cause appears to be a blend of genes and environment. "Genes alone do not cause the disease, and environment alone cannot cause the disease," Swaroop said.

In the new studies, researchers examined the genetic makeup of hundreds of people with and without age-related macular degeneration. Those with the condition turned out to share a particular variant of a gene known as LIPC, which regulates the metabolism of high-density lipoprotein (HDL) cholesterol in the blood.

But what is the link, exactly? It's not clear if high levels of HDL ("good") cholesterol might somehow boost a person's risk of developing the eye disease. That would create a complication if treatments designed to prevent the eye disease harmed cardiovascular health by lowering levels of "good" cholesterol. The reverse -- a treatment that boosted HDL cholesterol but also raised the risk of the eye disease -- would be a problem, too.

In any case, the researchers believe there isn't a direct cause-and-effect relationship, in which the genes in question cause both high levels of good cholesterol and raise the risk of the eye disease.

It's possible that the eye disease might have something to do with how the body transports nutrients in the blood, said Dr. Johanna Seddon, co-author of the other study and director of Tufts University Medical Center's Ophthalmic Epidemiology and Genetics Service.

Or, she said, a third factor might be involved.

In the big picture, said Swaroop, "we know today a lot more than we knew last month and last year." But that doesn't mean a better treatment -- or a cure -- for the eye disease is around the corner.

The studies are published in the April 12-16 online edition of the Proceedings of the National Academy of Sciences.